1. Field of the Invention
This invention relates to a fundus camera in which alignment light is projected onto a subject's eye, the alignment light reflected by the eye is converged on a point of a photographic optical path which is conjugate with a fundus of the eye, and a camera body is adjusted with respect to the eye by receiving the reflected alignment light.
2. Description of the Prior Art
Conventionally, a fundus camera is known in the prior art which is shown in FIG. 1. In FIG. 1, 1 denotes an observation light source, 2 denotes a condensing lens, 3 denotes a dichroic mirror, 4 denotes a plate with a ring-shaped slit, 5 denotes a relay lens, and 6 denotes a perforated mirror. The dichroic mirror 3 has an optical characteristic in that it transmits visible light and reflects infrared light.
Illumination light emitted by the observation light source 1 is condensed by the condensing lens 2 and is reflected by the dichroic mirror 3. The light reflected by the dichroic mirror 3 is infrared light. The infrared light is guided to a ring-shaped opening 4a of the ring-shaped slit plate 4. The infrared illumination light which has passed through the ring-shaped opening 4a is guided to the perforated mirror 6 via the relay lens 5. The infrared illumination light forms an image in the vicinity of the perforated mirror 6. The infrared illumination light reflected by the perforated mirror 6 goes toward an objective lens 7 disposed in the photographic optical path. The Infrared illumination light again forms an image in the vicinity of a cornea C of the eye E, and the fundus Ef is illuminated with the infrared illumination light passing through the cornea C.
The reflected light from the fundus Ef or retina is caused to form an image as an aerial image by means of the objective lens 7 at a point conjugate with the fundus Ef. After that, the reflected light passes through an opening of the perforated mirror 6 and is guided to a field lens 10 via a photographic lens 8 and a quick return mirror 9. The reflected light is again caused to form an aerial image in the vicinity of the field lens 10. And the reflected light is received on a light-sensive face of an image pickup tube 13, such as a CCD or a TV camera, via a reflecting mirror 11 and a relay lens 12. The image pickup tube 13 is connected to a monitor television system, and thereby an image Ea of the fundus Ef is displayed on a monitor 14.
End portions P, Q of two light guides 16, 17, such as an optical fiber, which serve to guide alignment light emitted by a light source 15 are disposed in the vicinity of the opening of the perforated mirror 6. A film 18 is disposed behind the quick return mirror 9. A shutter, not shown, is disposed between the film 18 and the quick return mirror 9. Alignment light for asking a small light spot as an alignment image on the cornea C is emitted from the end portions P, Q. The alignment light is guided to the cornea C through the objective lens 7 and is reflected by the cornea C. The reflected light from the cornea C is converged by the objective lens 7 on a point substantially conjugate with the fundus Ef and forms a small alignment image at the point. And then the reflected alignment light is guided to the image pickup tube 13 via the same optical path as the reflected light for forming the fundus image (retina image). Thereby, two alignment images are formed on the image pickup tube 13. Accordingly, two small light spots Pa, Qa which are each an alignment image are displayed on a screen of the TV monitor 14.
A photographer (operator) observes the retina image Ea displayed on the screen of the TV monitor 14 while focusing the image Ea to select a portion of the fundus to be photographed. Further, the photographer judges whether the light spots Pa, Qa are in focus and are normally positioned. According to the judgment, the correlative locations of the camera body and the subject's eye are determined. In the conventional fundus camera, a photographic light source 19 and a condenser lens 20 are disposed behind the dichroic mirror 3. When a photograph is taken, the photographic light source 19 is caused to emit light. The photographic light emitted by the photographic light source 19 passes through the condenser lens 20 and the dichroic mirror 3 and illuminates the eye fundus Ef with the visible illumination light. The light reflected by the eye fundus Ef forms an image on the film 18.
Conventionally, a fundus camera is also known in which the eye fundus is photographed in variable power. Such power variation is useful to examine the eye. However, the variation of a photographic lens from low power to high power during the observation of the eye fundus causes a field of view for observation (a field of view for photography) to be narrowed. For this reason, the light spots Pa and Qa are placed out of the field of view and therefore the positions of the camera body and the eye E cannot be determined.
To resolve this problem, an optical system of a fundus camera shown in FIG. 2 is known. In this optical system, a focussing lens 21 is disposed between the perforated mirror 6 and the quick return mirror 9. Further, photographic lenses 22a, 22b, each variable power lenses are interchangeably disposed in a photographic optical path behind the quick return mirror 9. Therefore, according to this conventional fundus camera, the light spots Pa and Qa are prevented from being placed out of the field of view since the alignment light is relayed to the image pickup tube 13 before the photographic lenses 22a and 22b. Further, a character synthesizing device 23 is disposed between the image pickup tube 13 and the monitor TV 14. Further, an electric contact 24 is attached to each of the photographic lenses 22a, 22b. The electric contact 24 serves to input information about the powers of the photographic lenses 22a, 22b in use to the character synthesizing device 23.
Let it be supposed that the photographic lens 22a is inserted in the photographic optical system and a photographic angle of view A (45.degree.) is displayed as shown in (a) of FIG. 3. When the photographic lens 22a is removed from the photographic optical path and the photographic lens 22b is inserted thereinto instead, the photographic power is varied from high to low. Accordingly, the picture displayed on the monitor TV 14 is changed from (a) of FIG. 3 to (b) of FIG. 3. That is, the character synthesizing device 23 causes not only a ring D representing a photographic field of view but also a numeral representing a photographic angle of view B (30.degree.) to be displayed on the monitor TV 14. In (b) of FIG. 3, C1 through C4 each designate a target. However, according to this conventional fundus camera, when a working distance W (see FIG. 1) between the eye and the camera body is improper, the light spots Pa, Qa which are alignment images grow faint and low in brightness as shown by Pa', Qa' of FIG. 4 as an enlarged view. Accordingly, the working distance W is adjusted so that the light spots Pa, Qa grow clear. However, according to the conventional fundus camera, if the working distance W is improper even slightly, the light spots Pa, Qa abruptly grow faint. Therefore, the working distance W cannot be easily adjusted. Further, alignment accuracy still depends on operator's skill.
Further, according to the fundus camera shown in FIG. 2 in which the magnification of an eye fundus image is varied, since its magnification in observation is different from that in photography, the photographer will have a sense of inconvenience. Further, since the variation in magnification of an eye fundus image from low to high inevitably leads to the variation in magnification of an alignment image, the alignment image grows low in brightness, hence it becomes difficult to clearly observe the alignment image.